1. Technical Field
The technical field of this invention is reduction of drag in fluid flows by coating surfaces with super hydrophobic materials.
2. Description of the Prior Art
Drag reduction during flow in conduits, for example of petroleum in pipelines, is an important economic goal with considerable business activity. For example, U.S. Pat. No. 6,162,773 by Eaton et al which issued Dec. 19, 2000 is one of many patents aimed at synthesizing high molecular weight polymers, usually polyalphaolefins, polyalkanes, fluorinated urethanes, etc. These polymers mix in the hydrocarbon fluid and adsorb energy during turbulent flow, reducing drag. Similar principles have been used to create drag reducing agents for water, e.g. U.S. Pat. No. 4,585,810 by Bock et al. which issued Apr. 29, 1986. In this patent a water soluble high molecular weight polymer is added to water to increase pumping rate of firehoses, sewer systems, etc. The subject of this patent is likewise reduction of energy during flow, for aqueous liquids, water and water-loving, hydrophilic, liquids, for example during pumping for irrigation, or water flow around boat hulls, pumping in various industrial processes, and filling of tight annulus during industrial processes such as making liquid crystal displays, filtration, etc. The method of drag reduction, however, is entirely different than in the drag reduction applications using polymers aforementioned. In the art of this patent a super hydrophobic coating is created on the interior conduit surface or the interior of the tight space or annulus or surface of a boat hull. The potential for drag reduction by super hydrophobic surfaces has been recognized and proven in the technical literature, e.g., Watanabe, et. al. (1996) found a 14% drag reduction in laminar flow in a pipe using a highly water-repellent coating of fluorine alkane modified acrylic resin with added hydrophobic silica. This coating had a contact angle with the wall surface of 150 degrees The coating reduces the contact area of liquid wetting drastically, thereby reducing friction and damping turbulent eddies.
U.S. Patent Application 20050061221 by Paszkowski published on Mar. 24, 2005 describes a fractal coating with geometry to cause super hydrophobicity. However a process for producing such a coating on a pipe wall or other object is not given. U.S. Patent Application 20050008495 by Wobben was published on Jan. 13, 2005 has the objective of reducing noise from wind turbine blades by forming a fluid (air)-repellent layer or surface on the wind turbine blades. The application claims the use of nanonails wherein the nanonails comprise hydrophobic polymers attached to the surface. These two patent applications teach that fluid drag can be reduced by hydrophobic coatings but does not provide a practical way to produce such coatings. On the other hand, the coating used by Watanabe is a practical coating, however, its hydrophobicity can be improved and longevity and other performance characteristics improved.
Super hydrophobic coatings of this type have recently been cited for the purpose of keeping surfaces cleaner, similar to the lotus plant, and the high contact angle is known as the “lotus effect”. However, the only materials so far commercialized to produce this cleaning effect, e.g. MINCOR from BASF, and or TEGOTOP from Degussa, have been tested by us and have been found to be unsuitable. When coated alone or mixed with various types of hydrophobic particles, these and many other polymers, e.g. acrylic resins, silicon containing graft copolymers, functional/non functional siloxanes, inorganic hybrids such as silsesquioxanes, acrylic polymers containing perfluoro pendant groups, TEFLON/NAFION type fluoro polymers, urethanes, fluorourethanes, polyethers, polyesters and silicon modified polyacrylates, it is found that the resulting coating is initially super hydrophobic and may remain so for long periods indoors. However, when exposed to outdoor UV light, rubbed even slightly, or in general exposed to weather, the coating loses super hydrophobicity (which we define as the instant shedding of water with no remaining drops) and becomes less hydrophobic within days or even hydrophilic and hence less useful for the object of this invention. Examination under the microscope after a week of exposure on a panel in a UV cabinet reveals that a coating made from fumed silica and at least one film forming binder as per U.S. Pat. No. 6,683,126 by Keller et al. issued in Jan. 27, 2004 is disintegrating as set forth in FIG. 1 wherein the polymers were selected from the class that have hydrophobicity such that the contact angle of the binders were above 90 degrees. (U.S. Pat. No. 6,683,126). Furthermore it has been published (GM reference) that lotus type coatings, when exposed to steam, lose their hydrophobicity. We have found that the state of the art is that no practical lotus coatings exist for the object of this invention.
Another important object of making a practical, UV stable coating is drag reduction for the skin of airplanes, and the blades of wind turbines. The super hydrophobic coating will also have a beneficial effect of reducing turbulent noise in these applications. For example, U.S. Patent Application 20050008495 by Wobben published on Jan. 13, 2005 claims coating a fluid repellent structure such as a shark skin structure with ribs or nanonails would reduce the is energy of air eddies and hence reduce noise. A drag reduction tape is made by 3M (RIBLET TAPE) for this purpose. Also micro-silicone paint (LOTUSAN) is mentioned for reducing icing.
U.S. Patent publication 20060110542 published on May 25, 2006 discloses a composition for forming a detachable and renewable protective coating produced by making a highly concentrated dispersion of hydrophobically modified silica particles in the presence of a disilazane derivative under high shear conditions.